The Query Labyrinth

Welcome back, @name! You stand at the gates of the Query Labyrinth - the most intricate part of the Great Library. Here I keep the scrolls that cannot be found with a single glance at a shelf. Sometimes, to find one book, you must first ask the Library another question, and only its answer points to the right shelf.

A question inside a question

In our world this trick is called a subquery. It is an ordinary SELECT query enclosed in round brackets and placed inside another query.

1SELECT tytul
2FROM ksiazki
3WHERE cena > (SELECT AVG(cena) FROM ksiazki);

First the Library runs the inner subquery - it computes the average price of all books. Only then does the outer query use that number to select books more expensive than the average.

Why descend into the Labyrinth?

• to compare a row with the result of a calculation (e.g. the average),
• to check whether a value belongs to a list returned by another query,
• to ask whether a related scroll exists on another shelf,
• to combine the results of two searches into one list.

What you will learn here

We will walk through every corridor of the Labyrinth: scalar subqueries, the IN operator, correlated subqueries, EXISTS / NOT EXISTS, derived tables in the FROM clause, and combining sets with UNION and UNION ALL.

Hold tight to Ariadne's thread - the logic of your query - and you will not get lost in the alleys. Let us begin!

@name